The present invention relates to an optical disk apparatus which records and reproduces information on an optical disk, and particularly to a laser driver which is useful for improving the quality of reproduced signal and to an optical disk apparatus which uses the laser driver.
Optical disk information reproduction apparatus based on the semiconductor laser adopts the high frequency superimposition scheme in which a high-frequency current is superimposed on a d.c. current for driving the semiconductor laser in order to reduce the laser noise which emerges due to the interference between the semiconductor laser and the reflected light from the recording medium. However, despite the use of the high frequency superimposition scheme, there was found a phenomenon of the presence of a significant residual laser noise. A study conducted recently revealed that this phenomenon is caused by the relaxation oscillation which arises when the semiconductor laser undergoes the high-frequency modulation such as the case of high frequency superimposition scheme, and that the resulting laser noise is dependent on the superimposed frequency. A method of setting the superimposed frequency in consideration of the laser noise caused by the relaxation oscillation is disclosed in Japanese Patent Unexamined Publication No. Hei 11-54826.
FIG. 7 shows a measurement result of the relation between the laser noise and the frequency and amplitude of the superimposed high-frequency current. The laser noise was managed to be within the allowable level by setting the superimposed current within region A or B on the graph of FIG. 7. However, the high frequency superimposing circuit has its operational efficiency falling as the frequency rises, resulting in an increased heat dissipation. The semiconductor laser operating at a higher temperature produces a larger laser noise in general, and therefore the region B located in the higher frequency range is not suitable for use. Another region C shown by hatching in FIG. 7 necessitates the reduction of unwanted radiation or electro-magnetic interference(EMI) by means of an expensive shielding structure to meet the regulation, and therefore this region is avoided. On this account, for the fulfillment of both the reduction of laser noise and the reduction of EMI, the superimposed current needs to be set within the region A and at the same time outside the region C, i.e., the superimposed current must be within range D in terms of frequency and within range E in terms of amplitude.
FIG. 4 shows by block diagram a laser driver which controls the superimposed current. The laser driver is made up of a semiconductor laser drive circuit 1, an oscillator control circuit 3 and oscillator 4 which form a high frequency superimposing oscillator, and an adder 6. At information reproduction, the semiconductor laser drive circuit 1 releases a laser driving d.c. current 2 and the adder 6 superimposes the output (high-frequency current 5) of the oscillator 4 to the laser driving d.c. current 2, so that a resulting current drives a laser diode 8. The oscillator 4 has its output frequency and amplitude controlled by the oscillator control circuit 3, which is responsive to an oscillator control signal 31 provided by an external microcomputer. Based on this function, the prior art method sets the frequency and amplitude of the superimposed current to meet the above-mentioned conditions.
However, this laser driver involves a problem of the disparity among individual devices, and some laser drivers having an invariable or same frequency setting are found to vary their output frequencies out of the intended frequency range D due to the disparity in frequency of the high-frequency current 5 produced by the oscillator 4. The frequency disparity of the high-frequency current 5, which is attributable to the semiconductor manufacturing process, is difficult to make smaller, giving rise to the need of frequency adjustment for the superimposed current before the shipment from the factory. This frequency adjusting process is based on the detection of a weak EMI of the laser driver with an antenna and the measurement of frequency with a spectrum analyzer, and it has been demanded the simplification of this intricate measuring facility.
Accordingly, it is an object of the present invention to provide a laser driver which is capable of measuring the superimposed frequency of semiconductor laser simply and accurately, and provide an optical disk apparatus which uses the laser driver.
Another object of the present invention is to provide an optical disk apparatus which is capable, within the apparatus, of controlling the superimposed frequency of semiconductor laser.
Specifically, the inventive optical disk apparatus comprises a semiconductor laser which projects a laser beam onto an optical disk, a semiconductor laser driver which drives the semiconductor laser with a current, with a high-frequency current being superimposed thereon, and measures the frequency of the superimposed current, and a main controller which controls the frequency of the superimposed current output by the semiconductor laser driver by using the frequency measured by the semiconductor laser driver.
The apparatus further includes a demodulation device which implements the error correction for reproduced data and the detection of error rate, and a data strobe device which detects the jitter emerging at the analog-to-digital conversion of the reproduced signal. The main controller uses the frequency measured by a frequency measuring circuit and the detected error rate or jitter to determine the frequency which minimizes the error rate or jitter, and controls the laser driver to produce a superimposed current of the determined frequency.
The inventive laser driver comprises a semiconductor laser drive circuit which outputs a d.c. current to a semiconductor laser, a high frequency superimposing oscillator which superimposes a high-frequency current on the output of the semiconductor laser drive circuit, and a frequency measuring circuit which measures the frequency of the superimposed current produced by the high frequency superimposing oscillator. The frequency measuring circuit is specifically made up of a digitizing circuit which converts the high-frequency current signal into a binary digital signal and a frequency demultiplying circuit which demultiplies the frequency of digital signal to produce a high frequency superimposition monitor signal. The frequency measuring circuit may further include a frequency counter which measures the frequency of the superimposed current based on a reference clock signal provided externally and the superimposition monitor signal.
The inventive laser control method, which is intended for an optical disk apparatus which drives a semiconductor laser with a high-frequency current to project a laser beam onto an optical disk thereby to reproduce data recorded on the optical disk, is designed to measure the frequency of the high-frequency current and control the frequency by using the measured frequency. Specifically, it detects the above-mentioned error rate or jitter and controls the semiconductor laser by using the detection result and measured frequency so that the error rate or jitter may be minimal.